Who mentioned the Bible?
The reason the hypothetical nature of your argument is irrelevant is because it does not happen. If a man thinks they are in the wrong body, it is because they have a mental illness. If you need to mutilate your body to fit some abstract feeling that you are experiencing, you are possessed by a destructive mentality. The causes of that mentality can be either environmental of physiological, or both, but either way it is counter-productive to the person for the people in their lives to play along with these destructive impulses. Instead we should try to understand why they are feeling this way and address the causes, and help the person feel more comfortable in their own skin, their real identity, instead of allowing them to try to rip that skin away because they think they “identify” as a skeleton.